1. Field of the Invention
This invention relates to a non-contact current meter for sensing electrical currents and, more particularly, to a hybrid non-contact clamp-on current meter utilizing a magnet core wherein the saturation measurement technique is utilized in a DC to low frequency mode and a current transformer technique is utilized when in a high frequency mode.
2. Description of the Background Art
Many types of magnetic core electric current measurement meters utilizing the saturation technique are known and are in wide use today throughout the electrical industry. However, none of the present current meters provide high accuracy measurements for DC to low frequency currents as well as provide the capability to accurately measure high frequency currents.
In general, non-contact clamp-on current meters provide a convenient means for measuring DC to low frequency or high frequency line currents in a current carrying conductor without a need to interrupt the circuit to insert the measuring device. The prior art non-contact clamp-on current meters are generally provided with a magnetic core that surrounds the conductor. The magnetic core is typically comprised of several sections that are pieced together so that the current carrying conductor can be measured. By the magnetic core encircling the current carrying conductor, a single turn winding has, in effect, been established on the magnetic core.
Generally, the non-contact current meters for measuring DC to low frequency currents utilize the hysteresis characteristic of a magnetic core to measure the line current in the current carrying conductor. In effect, the line current to be measured in the current carrying conductor acts to magnetically adjust the magnetic core by way of coupling the magnetic field generated by the line current. The magnetic core generally includes sensing windings positioned thereon of which carry an excitation current of a general fundamental frequency. This excitation current of fundamental frequency is generally used to saturate the magnetic core prior to the introduction of a line current to be measured. The magnetic fields associated with the excitation current are, hence, coupled to the magnetic core wherein a magnetic flux is produced therein and caused to flow. The magnetic flux then induces an AC output voltage on the output terminals of which is a second order or higher even harmonic of the fundamental frequency. When no line current is flowing in the current carrying conductor the AC output voltage detected on the output terminals is zero. Upon the introduction of a line current in the current carrying conductor, the AC output voltage changes and the detected value is proportional to the amplitude and polarity of the line current being measured.
Due to the nature of the saturation technique used for measuring DC to low frequency currents, it follows that the accuracy of the line current measurements greatly depends upon the type of material used, the shape, and the particular construction of the magnetic core. The shape of the magnetic core and the continuity existing in the path of which the magnetic flux flows within the magnetic core greatly affects the efficiency of the magnetic core.
In the magnetic cores utilized in the prior art, the magnetic flux paths are formed from individual sections that are pieced together, having many discontinuities therein. These discontinuities in the flow path of the magnetic flux resultingly magnify the problems with zero errors in the measurements. In essence, any discontinuities in the saturating path of the magnetic core will act as a gap and, therefore, require more drive signal to achieve magnetic core saturation. Additionally, the shape of the prior art magnetic cores often served not to achieve maximum saturation. Representative current meters are disclosed in U.S. Pat. Nos. 3,323,056, 3,490,042, 4,118,597, 4,243,931, 4,266,190, 4,274,051, 4,274,052, 4,278,939, 4,286,211, 4,309,655, 4,362,990, 4,482,862, 4,529,931, 4,914,381, 4,914,383, 5,008,612, and 5,307,008, the disclosures of which are hereby incorporated by reference herein.
Typical examples of non-contact clamp-on type magnetic cores that have been used in the prior art are seen in Bergh, U.S. Pat. No. 3,007,106, and Harnden, U.S. Pat. No. 4,471,300. In the Bergh patent, the magnetic core is comprised of many pieced together portions that essentially introduce many discontinuities in the magnetic core as well as provide a non-ideal magnetic flux path. The magnetic core taught in the Bergh patent is of a general square shape that limits the amount of flux saturation that can be achieved and, thus, requires more power to saturate the magnetic core and is significantly less accurate than the present invention.
In the Harnden patent, the magnetic core is comprised generally of a circular toroidal loop having a hinge in one section and a pin and groove type of securing means in an opposite section of the magnetic core. This particular magnetic core, as taught by Harnden, operates as a current transformer in principle and does not utilize a saturation technique for measurements. Additionally, the Harnden patent is limited in that it is incapable of measuring Direct Current. Further, due to the construction of the Harnden magnetic core, the accuracy achievable from the Harnden current meter is inferior to the present invention.
In addition, generally, the prior art magnetic cores, as utilized currently in the industry, are expensive to manufacture and require the assembling of various portions. Moreover, the magnetic cores currently being used have inherent aspects that contribute to significant inaccuracies in current measurements. Moreover, none of the prior art current meters can mesure both DC to low frequency currents as well as high frequency currents that extend well into the MHZ range with resultingly high accuracy, the range being limited only by the core material.
Therefore, it is an object of this invention to provide improvements to overcome the aforementioned inadequacies of the prior art magnetic cores and provide an improvement which is a significant contribution to the advancement of the current sensing art.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter which utilizes a magnetic core constructed from only two conventional toroidal cores.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter which is comprised of two toroidal cores that are void of discontinuities within the flow path of the magnetic flux so as to minimize inaccuracies in line current measurements and provide a low power requirement during operation in the DC to low frequency mode of operation utilizing the saturation technique.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter of which two toroidal cores are utilized having a circular shape so that a more thorough and optimum flux saturation is achieved therein thereby reducing the problems associated with zero errors when operating in the DC to low frequency mode utilizing the saturation technique.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter which switches to a current transformer mode of operation when sensing a high frequency current.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter which utilizes a first and second set of windings electrically coupled in a bridge configuration as positioned on the magnetic core in both of the DC to low frequency and high frequency operating modes.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter that includes a switching means for changing the operating mode of the current meter according to whether a DC/low frequency signal is being measured or whether a high frequency signal is being measured.
Another object of this invention is to provide a hybrid non-contact clamp-on current meter whereby the magnetic core utilized is easy and inexpensive to manufacture in that it can be cut from a conventional toroidal ferrite core and that no piece assembly is necessary.
Another object of this invention is to provide a current meter for measuring the flow of line current through a current carrying conductor, the current meter comprising in combination: a magnetic core means for electrically coupling and sensing the line current flowing through the current carrying conductor; a first set of windings positioned on the magnetic core means; a second set of windings positioned on the magnetic core means, the second set of windings being electrically coupled to the first set of windings, the first and second sets of windings further including a pair of input and output terminals; a drive means for providing an excitation current to the first and second sets of windings, the drive means being electrically coupled to the pairs of input and output terminals; and a switching means for changing the mode of operation of the current meter from low frequency measurement mode to high frequency measurement mode, the switching means being electrically coupled to the drive means and the output terminals.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the summary of the invention, and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.